There is known a radio-frequency communication device including a transmission section that transmits a predetermined transmitted signal via a transmission antenna and a reception section that receives, via a reception antenna, a reply signal returned in response to the transmitted signal. For example, there is illustrated a radio-frequency tag communication device (an interrogator) of an RFID (Radio-Frequency Identification) system that reads out information from a compact radio-frequency tag (a transponder) storing predetermined information, in a noncontacting manner. In the RFID system, even when the radio-frequency tag is dusty or located out of sight, communication between the radio-frequency communication device and the radio-frequency tag allows read-out of information stored in the tag. Thus, practical application of the system is expected in various fields such as product management and inspection processes.
Meanwhile, usually, the radio-frequency tag communication device transmits a predetermined transmitted signal (an interrogating wave) toward the radio-frequency tag via an antenna, as well as receives, via an antenna, a reply signal (a response wave) returned from the radio-frequency tag receiving the transmitted signal. In this manner, the communication device communicates information with the radio-frequency tag. Sometimes, the received reply signal includes a strong leakage signal (a direct wave) mixed from a transmission side, thereby resulting in increase in the entire signal strength of the received signal. Increased signal strength exceeds allowable amplifier input strength, thus inhibiting an amplifier from providing its sufficient amplifying effect. As a result, the amplifier cannot sufficiently amplify a reply signal component from the radio-frequency tag. Accordingly, there has been a problem of reducing an SN ratio (a signal-to-noise ratio).
In view of this, there is proposed a technique for eliminating a leakage signal from the transmission side, such as a carrier phase noise suppressing circuit described in Patent Literature 1. The technique generates a cancel signal having a frequency and a signal strength equal to those of a carrier component and a phase opposite to that of the carrier component, and then adds the cancel signal to a received signal to input to a reception circuit. This renders a signal having a large signal-to-noise ratio, thereby increasing reception sensitivity.
[Patent Literature 1] JP-A-1998-62518
[Patent Literature 2] JP-A-1996-122429